/* =========================================

   Filename:  sum_recursive.c
   Filedate:  January 18, 2007

   The program asks for  positive integer  n 
   and then computes the following sum:  

    2 + 3 + 6 + ... + (n*n - 2*n + 3)

   The computation uses recursion.

   =========================================  */

#include<stdio.h>
#include<stdlib.h>

/*  =====   Declarations ======  */ 

int sum_up(int n);

/*  =====   Definitions  ======  */ 

int sum_up(int n) {
  if (n == 1) {
    return 2;
  }
  else {
    return(n*n - 2*n + 3 + sum_up(n-1));
  }
}

/*  =====   Main  ======  */ 

int main() {

  int n;

  printf("\nInput a positive integer n.\n\n");
  scanf("%d", &n);

  printf("\nWhen  n  =  %d,\n\n",n);
  printf("2 + 3 + 6 + ... + (n*n - 2*n + 3)\n\n"); 
  printf("is:  %d\n", sum_up(n));
  return(0);
}